1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU%.%2!,G0G0!30%#43G0G0/ further amended)1 General characteristics1.1 There should be a 4th-order bit rate of 139 264 kbit/s in the digital hierarchy which is based on the 2nd-orderbit rate of 8448 kbit/s.There s
2、hould be two methods of achieving the 4th-order bit rate:Method 1 - by using a 3rd-order bit rate of 34 368 kbit/s in the digital hierarchy.Method 2 - by directly multiplexing sixteen digital signals at 8448 kbit/s.The digital signals at the bit rate of 139 264 kbit/s obtained by these two methods s
3、hould be identical.1.2 The existence of the above two methods implies that the use of the bit rate of 34 368 kbit/s should not beimposed on an Administration that does not wish to realize the corresponding equipment.1.3 In accordance with the above two methods, the following realizations of digital
4、multiplex equipments usingpositive justification are recommended:Method 1 - Realization by separate digital multiplex equipments: one type which operates at 34 368 kbit/s andmultiplexes four digital signals at 8448 kbit/s; the other type which operates at 139 264 kbit/s and multiplexesfour digital s
5、ignals at 34 368 kbit/s.The multiplexing for the 34 368 kbit/s digital multiplex equipment is recommended in 1.4 below, whilefurther specification of this equipment is given in 2 below.The multiplexing for the 139 264 kbit/s digital multiplex equipment is recommended in 1.5 below, whilefurther speci
6、fication of this equipment is given in 3 below.Method 2 - Realization by a single digital multiplex equipment which operates at 139 264 kbit/s and multiplexessixteen digital signals at 8448 kbit/s.The digital multiplexing for the 139 264 kbit/s bit rate should be achieved by multiplexing, in accorda
7、nce with 1.5 below, four digital signals at 34 368 kbit/s, each of which is obtained by multiplexing, in accordance with 1.4 below, four digital signals at 8448 kbit/s. Further specification of this equipment is given in 4 below.1.4 Multiplexing four digital signals at 8448 kbit/s1.4.1 Bit rateThe n
8、ominal bit rate should be 34 368 kbit/s.The tolerance on that rate should be 20 parts per million (ppm).1.4.2 Frame structureTable 1/G.751 gives:- the tributary bit rate and the number of tributaries;2 Fascicle III.4 - Rec. G.751- the number of bits per frame;- the bit numbering scheme;- the bit ass
9、ignment;- the bunched frame alignment signal.TABLE 1/G.75134 368 kbit/s multiplexing frame structureTributary bit rate (kbit/s) 8448Number of tributaries 4Frame structure Bit numberFrame alignment signal (1111010000)Alarm indication to the remote digital multiplex equipmentBit reserved for national
10、useBits from tributariesJustification service bits Cj1(see Note)Bits from tributariesJustification service bits Cj2(see Note)Bits from tributariesJustification service bit Cj3(see Note)Bits from tributaire available for justificationBits from tributariesSet I1 to 10111213 to 384Set II1 to 45 to 384S
11、et III1 to 45 to 384Set IV1 to 45 to 89 to 384Frame lengthBits per tributaryMaximum justification rate per tributaryNominal justification ratio1536 bits378 bits22 375 kbit/s0.436Note - Cjn, indicates the nth justification service bit of the jth tributary.1.4.3 Loss and recovery of frame alignmentLos
12、s of frame alignment should be assumed to have taken place when four consecutive frame alignment signalshave been incorrectly received in their predicted positions.When frame alignment is assumed to be lost, the frame alignment device should decide that such alignment haseffectively been recovered w
13、hen it detects the presence of three consecutive frame alignment signals.Fascicle III.4 - Rec. G.751 3The frame alignment device having detected the appearance of a single correct frame alignment signal, shouldbegin a new search for the frame alignment signal when it detects the absence of the frame
14、 alignment signal in one of thetwo following frames.Note - As it is not strictly necessary to specify the detailed frame alignment strategy, any suitable framealignment strategy may be used provided the performance achieved is at least as efficient in all respects as that obtainedby the above frame
15、alignment strategy.1.4.4 Multiplexing methodCyclic bit interleaving in the tributary numbering order and positive justification is recommended. Thejustification control signal should be distributed and use the Cjn-bits (n = 1, 2, 3, see Table 1/G.751). Positivejustification should be indicated by th
16、e signal 111, no justification by the signal 000. Majority decision is recommended.Table 1/G.751 gives the maximum justification rate per tributary and the nominal justification ratio.1.4.5 Service digitsTwo bits per frame are available for service functions. Bit 11 of Set I is used to transmit an a
17、larm indication tothe remote multiplex equipment when specific fault conditions are detected in the multiplex equipment (see 2.5 and4.5 below). Bit 12 of Set I is reserved for national use. On a digital path crossing the border, this bit is fixed at 1.1.5 Multiplexing four digital signals at 34 368
18、kbit/s1.5.1 Bit rateThe nominal bit rate should be 139 264 kbit/s. The tolerance on that rate should be 15 parts per million (ppm).1.5.2 Frame structureTable 2/G.751 gives:- the tributary bit rate and the number of tributaries;- the number of bits per frame;- the bit numbering scheme;- the bit assig
19、nment;- the bunched frame alignment signal.1.5.3 Loss and recovery of frame alignmentLoss of frame alignment should be assumed to have taken place when four consecutive frame alignmentsignals have been incorrectly received in their predicted positions.When frame alignment is assumed to be lost, the
20、frame alignment device should decide that such alignment haseffectively been recovered when it detects the presence of three consecutive frame alignment signals.The frame alignment device having detected the appearance of a single correct frame alignment signal, shouldbegin a new search for the fram
21、e alignment signal when it detects the absence of the frame alignment signal in one of thetwo following frames.Note - As it is not strictly necessary to specify the detailed frame alignment strategy, any suitable framealignment strategy may be used provided the performance achieved is at least as ef
22、ficient in all respects as that obtainedby the above frame alignment strategy.1.5.4 Multiplexing methodCyclic bit interleaving in the tributary numbering order and positive justification is recommended. Thejustification control signal should be distributed and use the Cjn-bits (n = 1, 2, 3, 4, 5, se
23、e Table 2/G.751). Positivejustification should be indicated by the signal 11111, no justification by the signal 00000. Majority decision isrecommended.Table 2/G.751 gives the minimum justification rate per tributary and the nominal justification ratio.4 Fascicle III.4 - Rec. G.751TABLE 2/G.751139 26
24、4 kbit/s multiplexing frame structureTributary bit rate (kbit/s) 34 368Number of tributaries 4Frame structure Bit numberFrame alignment signal (111110100000)Alarm indication to the remote digital multiplex equipmentBits reserved for national useBits from tributariesJustification service bits Cjn (n
25、= 1 to 4) (see Note)Bits from tributariesJustification service bit Cj5(see Note)Bits from tributaire available for justificationBits from tributariesSet I1 to 121314 to 1617 to 488Set II to V1 to 45 to 488Set VI1 to 45 to 89 to 488Frame lengthBits per tributaryMaximum justification rate per tributar
26、yNominal justification ratio2928 bits723 bits47 563 bit/s approx.0.419Note - Cjnindicates the nth justification service bit of the jth tributary.1.5.5 Service digitsFour bits per frame are available for service functions. Bit 13 of Set I is used to transmit an alarm indication tothe remote multiplex
27、 equipment when specific fault conditions are detected in the multiplex equipment (see 3.5 and4.5 below). Bits 14 to 16 of Set I are reserved for national use. On a digital path crossing the border, these bits are fixedat 1.2 Digital multiplex equipment operating at 34 368 kbit/s and multiplexing fo
28、ur tributaries at 8448 kbit/s2.1 MultiplexingThe multiplexing for the 34 368 kbit/s bit rate should be in accordance with 1.4.2.2 Digital interfacesThe digital interfaces at 8448 kbit/s and 34 368 kbit/s should be in accordance with Recommendation G.703.Fascicle III.4 - Rec. G.751 52.3 Jitter2.3.1 J
29、itter transfer characteristicAn 8448 kbit/s signal, modulated by sinusoidal jitter, should be subject to a muldex jitter transfer characteristicwithin the gain/frequency limits given in Figure 1/G.751. The equivalent binary content of the test signal should be1000.2.3.2 Tributary output jitterThe pe
30、ak-to-peak jitter at a tributary output in the absence of input jitter should not exceed 0.25 UI whenmeasured in the frequency range up to 400 kHz.When measured with an instrument incorporating a bandpass filter having a lower cutoff frequency of 3 kHz, arolloff of 20 dB/decade and an upper limit of
31、 400 kHz, the peak-to-peak output jitter should not exceed 0.05 UI with aprobability of 99.9% during a measurement period of 10 s.Note - For interfaces meeting the national low Q option detailed in Recommendation G.703, the lower cutofffrequency for the above measurement should be 80 kHz.Note 1 - Th
32、e frequency f0should be less than 20 Hz and as low as possible (e.g. 10 Hz),taking into account the limitations of measuring equipment.Note 2 - To achieve accurate measurements, the use of a selective method is recommendedwith a bandwidth sufficiently small referred to the relevant measurement frequ
33、ency, but notwider than 40 Hz.Note 3 - The need to tolerate spurious responses greater than -19.5 dB in the frequencyrange f6to f7is for further study.FIGURE 1/G.7512.3.3 Multiplex signal output jitterIn the case where the transmitting timing signal is derived from an internal oscillator, the peak-t
34、o-peak jitter atthe 34 368 kbit/s output should not exceed 0.05 UI when it is measured within the frequency range from f1= 100 Hz tof4 = 800 kHz.6 Fascicle III.4 - Rec. G.7512.4 Timing signalIf it is economically feasible, it may be desirable to be able to derive the multiplexer timing signal from a
35、nexternal source as well as from an internal source.2.5 Fault conditions, and consequent actions2.5.1 Fault conditionsThe digital multiplex equipment should detect the following fault conditions:2.5.1.1 Failure of power supply.2.5.1.2 Loss of an incoming signal at 8448 kbit/s at the input of the mul
36、tiplexer.Note - Where separate circuits are used for the digital signal and the timing signal then loss of either or bothshould constitute loss of the incoming signal.2.5.1.3 Loss of the incoming signal at 34 368 kbit/s at the input of the demultiplexer.Note - The detection of this fault condition i
37、s required only when it does not result in an indication of loss offrame alignment.2.5.1.4 Loss of frame alignment.2.5.1.5 Alarm indication received from the remote multiplex equipment at the 34 368 kbit/s input of the demultiplexer(see 2.5.2.2 below).2.5.2 Consequent actionsFurther to detection of
38、a fault condition, actions should be taken as specified by Table 3/G.751. The consequentactions are as follows:2.5.2.1 Prompt maintenance alarm indication generated to signify that performance is below acceptable standards andmaintenance attention is required locally. When the Alarm Indication Signa
39、l (AIS) (see Note 2 under 2.5.2.5) at34 368 kbit/s is detected at the input of the demultiplexer, the prompt maintenance alarm indication associated with lossof frame alignment should be inhibited, while the rest of the consequent actions are in accordance with those associatedin Table 3/G.751 with
40、the fault condition.Note - The location and provision of any visual and/or audible alarm activated by this maintenance alarmindication is left to the discretion of each Administration.2.5.2.2 Alarm indication to the remote multiplex equipment generated by changing from the state 0 to the state1 bit
41、11 of Set I at the 34 368 kbit/s output of the multiplexer.2.5.2.3 AIS (see Notes 1 and 2 below) applied to all four 8448 kbit/s tributary outputs from the demultiplexer.2.5.2.4 AIS (see Notes 1 and 2 below) applied to the 34 368 kbit/s output of the multiplexer.2.5.2.5 AIS (see Note 2 below) applie
42、d to time slots of the 34 368 kbit/s signal at the output of the multiplexer,corresponding to the relevant 8448 kbit/s tributary.The method of transmitting the AIS at the output port of the multiplexer in time slots corresponding to a faultyinput tributary should be such that the status of the justi
43、fication control digits is controlled so as to ensure that the AIS iswithin the tolerance specified for that tributary.Note 1 - The bit rate of the AIS at the output of the multiplexer equipment or at the output of the demultiplexerequipment should be in accordance with the interface specifications.
44、Fascicle III.4 - Rec. G.751 7Note 2 - The equivalent binary content of the AIS (AIS) at 8448 kbit/s and 34 368 kbit/s is nominally acontinuous stream of 1s. The strategy for detecting the presence of the AIS should be such that the AIS is detectableeven in the presence of an error ratio 1 10-3. Howe
45、ver a signal with all bits except the frame alignment signal in the 1state, should not be mistaken as an AIS.2.5.3 Time requirementsThe fault detection and the application of the consequent actions given in 2.5.2.2 to 2.5.2.5, including thedetection of AIS, should be completed within a time limit of
46、 1 ms.TABLE 3/G.751Fault conditions and consequent actionsConsequent actions (see 2.5.2 or 3.5.2)AIS appliedEquipmentpartFault condition(see 2.5.1 or 3.5.1)PromptmaintenancealarmindicationgeneratedAlarmindication tothe remotemultiplexequipmentgeneratedTo all thetributariesTo thecompositesignalTo the
47、 relevanttime slotsof thecompositesignalMultiplexeranddemultiplexerFailureof power supplyYes Yes, ifpracticableYes, ifpracticableMultiplexeronlyLoss of incoming signalon a tributaryYes YesLoss of incoming signal Yes Yes YesDemultiplexer Loss of frame alignment Yes Yes YesonlyAlarm indicationreceived
48、 fromthe remote multiplexequipmentNote - A Yes in the table signifies that a certain action should be taken as a consequence of the relevant fault condition. An openspace in the table signifies that the relevant action should not be taken as a consequence of the relevant fault condition, if thiscond
49、ition is the only one present. If more than one fault condition is simultaneously present the relevant action should be takenif, for at least one of the conditions, a Yes is defined in relation to this action.3 Digital multiplex equipment operating at 139 264 kbit/s and multiplexing four tributaries at34 368 kbit/s3.1 MultiplexingThe multiplexing for the 139 264 kbit/s bit rate should be in accordance with 1.5 above.8 Fascicle III.4 - Rec. G.7513.2 Digital interfacesThe digital interfaces at 34 368 kbit/s and 139 264 kbit/s should be in accordance
